The present invention relates to a process for the production of chlorine dioxide from an alkali metal chlorate, a mineral acid and a reducing agent. The process is carried out in a vessel operated under subatmospheric pressure, whereby water is evaporated and withdrawn together with chlorine dioxide and the alkali metal salt of the mineral acid is crystallized within the reaction vessel and withdrawn therefrom. According to the invention the precipitated alkali metal sulphate is contacted with a saturated water solution of alkali metal sulphate, whereafter neutral alkali metal sulphate is obtained. A water solution of this alkali metal sulphate is brought to an electrochemical membrane cell in which sulphuric acid is formed in the anode compartment and alkali metal hydroxide in the cathode compartment.
Chlorine dioxide used as an aqueous solution is of considerable commercial interest, mainly in pulp bleaching but also in water purification, fat bleaching, removal of phenols from industrial wastes, etc. It is therefore desirable to provide processes by which chlorine dioxide can be efficiently produced.
Chlorine dioxide is prepared by reduction of chlorate with a reducing agent in acid media. The most common reducing agents are chloride ions, sulphur dioxide and methanol and the processes can be summarized according to the following formulas: EQU NaClO.sub.3 +NaCl+H.sub.2 SO.sub.4 .fwdarw.ClO.sub.2 +1/2 Cl.sub.2 +H.sub.2 O+Na.sub.2 SO.sub.4 2 NaClO.sub.3 +SO.sub.2 .fwdarw.2 ClO.sub.2 +Na.sub.2 SO.sub.4 EQU 6 NaClO.sub.3 +CH.sub.3 OH+4 H.sub.2 SO.sub.4 .fwdarw.6 ClO.sub.2 +CO.sub.2 +5 H.sub.2 O+2 Na.sub.3 H(SO.sub.4).sub.2
The chlorate ions are provided by alkali metal chlorate, preferably sodium chlorate, the chloride ions by alkali metal chloride, preferably sodium chloride, or by hydrogen chloride, and the hydrogen ions are provided by mineral acids, generally sulfuric acid and/or hydrochloric acid.
These processes can be run at a normal pressure and usually the acid strength is high in the reaction medium to obtain sufficient efficiency in the process. Because of the high acid strength of the reaction medium the spent acid liquor from the reactor will consist of alkali metal sulphate in strong sulphuric acid. This spent acid liquor is a waste product which it is necessary to take care of. Depositing the spent acid liquor in the waste water system is neither possible nor desirable as the environmental demands do not permit depositing and the acid content of the liquor then gets lost.
It is also known to run the above mentioned processes at a subatmospheric pressure by using a combined reactor/evaporator. Alkali metal chlorate, reducing agent and mineral acid solutions are fed to a single vessel generator-evaporator-crystallizer in proportions sufficient to generate chlorine dioxide and chlorine, at a temperature of from about 50 to about 100 degrees centigrade, and an acidity of from about 2 to about 12 normal, with or without a catalyst. Water is removed by vacuum-induced evaporation at about 100 to 400 millimeters of mercury absolute, with concurrent withdrawal of chlorine dioxide and chlorine. The salt of the mineral acid is crystallized within the generator and the crystals withdrawn from the vessel. These crystals contain more or less acid depending on the acid strength of the reaction medium. It is only when the acid strength is below 4.8 N that a neutral sulphate is obtained, otherwise the precipitated crystals consist of sesquisulphate (Na.sub.3 H(SO.sub.4).sub.2) or bisulphate (NaHSO.sub.4).
The usual way to take care of the sulphate product has hitherto been to use it for tall oil splitting and as a "make up" in the cellulose plant, but in the latter case the acid content of the sulphate has not been utilized, but the sulphate crystals have been neutralized. Another field of use for the neutral sulphate waste product has been as a filler in detergents. However, the amount of sulphate used in both these fields decreases steadily because of change of production processes.
One advantageous way of utilize the spent liquor from the chlorine dioxide process is described in U.S. Pat. No. 4,129,484. This patent describes a process in which the spent acid liquor from a chlorine dioxide reactor is brought to an electrochemical membrane cell. According to one embodiment the cell comprises, besides the anode and cathode, at least one anion selective membrane and one cation selective membrane. Then at the electrolysis sodium hydroxide and hydrogen gas is formed in the cathode compartment and sulphuric acid and oxygen gas in the anode compartment. Thus only valuable chemicals are produced. The acid can be brought back to the chlorine dioxide reactor and the hydroxide can be used in the cellulose plant for example.
Although the manner of dealing with the sulphate residue according to the process in U.S. Pat. No. 4,129,484 is favourable it appeared that the process had certain draw backs. The spent acid liquor contains besides sulphate also chlorate and chloride ions and the solution is often very acidic. When this liquor is brought to the cell the chlorate ions will also migrate to the anode compartment and will there easily be transformed to chlorine dioxide in the acidic solution. Also chloride ions will be transformed to chlorine gas. Thus the reaction gas from the anode compartment will be a mixture of chlorine dioxide, chlorine and oxygen in stead of pure oxygen. It also appeared that it was difficult to run the electrochemical membrane cell for a time of any substantial length. After a while the voltage tended to rise, resulting in increased energy consumption and the membrane stopped to function. The reason for this was precipitations on the cathode and in the cathode membrane due to the use of the spent acid liquor. This liquor besides the sulphate and chlorate ions also often contains different metal ions such as Ca, Fe, Mg, Cr etc. being impurities emanating from the process water. At the electrolysis hydroxide ions are formed on the cathode surface and hence the solution on the surface will be very basic. Then the hydroxides of the metal impurities mentioned above will easily be precipitated. The same will also happen in the cathode membrane which will ultimately break down.